Most military vehicles that are employed with tactical units and transmit and receive communications by high frequency (HF) radio typically utilize tapered flexible vertical antennas called "whips". The most common whips consist of four 4 foot sections (16 foot whip) for use while the vehicle is moving, or eight 4 foot sections (32 foot whip) for use when the vehicle is stationary, referred to as "at-halt" operation. The sections are typically disconnected and stored in a canvas bag or the like during non-use, and are joined with threaded fittings during use. The bottom section has a threaded fitting for attachment to an antenna mount which is attached to a vehicle or shelter.
The vertical orientation of a whip is practical for vehicle mounting and useful for short distance ground wave (also known as surface wave) communications. However, certain intermediate distance communications requires that NVIS (near vertical incidence skywave) propagation modes be employed.
NVIS propagation involves refraction of radiated radio signals off the ionosphere at angles near 90.degree. above the horizontal. High frequency radio signals emitted at high vertical radiation angles are reflected/refracted from the ionosphere at acute angles and return to earth at short and medium distances with usable signal intensity. As a practical matter, NVIS propagation can only be accomplished within the high frequency radio portion of the radio spectrum (about 2-30 MHz). The best results are achieved within the lower frequency portion of the HF band (2-14 MHz). NVIS propagation using the ionosphere cannot be employed with frequency signals greater than about 20 MHz.
NVIS is particularly effective where the participating net stations are spread over geographical areas within approximately 300 miles of each other. For example, if HF radio stations operating on lower HF frequencies (2 to 14 MHz) radiate signals at between 90 degrees (directly overhead) to approximately 45 degrees, the signals will return to earth with considerable strength out to approximately 300 miles (480 kilometers) of the transmitting station.
To produce adequate signal levels at these high angles, sending and receiving antennas optimally should be horizontally polarized. However, the whip antenna, in its normal position, is vertically polarized, i.e., the electrostatic field is perpendicular to the Earth and the electromagnetic field is parallel to the Earth, thus producing low signal levels at high angles. The vertical radiation pattern of a vertical whip operating in the HF radio band has the highest gain at angles below 45 degrees above the horizon. The antenna thus performs fairly well when used for ground wave communication (short distances, usually under 25 miles), but poorly at high radiation angles necessary for NVIS communication.
One method of providing some horizontal polarization for NVIS operation is to bend the whip from the vertical toward the horizontal position to the maximum extent possible. However, because the bottom whip sections are rigid and spring mounts (when used) are stiff, it is difficult to bend the lower sections of the whip to a near-horizontal position where maximum current and radiation occurs. Also, lower whip sections and springs often break when bent too far.
Adjustable antennas for vehicles are known in the art. U.S. Pat. Nos. 4,109,251, 4,243,989, 4,827,273, 4,101,897, 4,055,845 and 4,074,271 each disclose an adjustable antenna mounted on a vehicle. However, the antennas used in the mountings disclosed in these patents are not for high frequency whip antennas. Further, these types of tiltable antennas are quite small and are intended for use at very high (VHF) and ultra high (UHF) frequencies which would normally support antennas no more than about 36 inches long. It would be impractical to use the types of antennas disclosed in the above-mentioned patents in place of the standard HF whip antenna. Also, systems for transmitting VHF and UHF frequency radio waves do not rely on and are not designed for reflection/refraction of such waves off the ionosphere. Therefore, these patents do not suggest tilting antennas for the purpose of utilizing NVIS propagation.
U.S Pat. Nos. 2,934,764, 2,979,729 and 4,625,213 each disclose mounts for antennas. The mounts hold an antenna to a surface in a fixed orientation, and do not provide for easy transition between vertical and horizontal polarization. Further, the disclosed mounts cannot in any way be substituted for a mount on an existing high frequency radio whip antenna connection. Also, none of the patented devices are practical for the exploitation of NVIS propagation phenomena peculiar only to HF radio communication, which is the intended use of the present invention.
There is therefore a need to provide a method and simple and inexpensive device for changing the polarization of an HF radio whip antenna between vertical and horizontal without requiring replacement of the vehicle's existing whip mount or other radio components.